1. Field of the Invention
The present invention relates to a device for reversal of an end-of-stitching chain or "end chain" in a two-needle overedging machine in which said end chain extending from an end edge of a sewn workpiece is formed about a primary tab provided parallel to the sewing direction on an inner side edge of a needle plate and exhibiting one free end oriented in the sewing direction, as well as about an auxiliary tab exhibited by the needle plate itself at a parallel side by side position relative to the primary tab and having one respective free end oriented in the sewing direction, said device comprising a suction chain-cutting member operatively disposed in the extension of the side edge of the needle plate, immediately downstream of said needle plate with reference to the sewing direction and arranged to draw in and cut the end chain for separating it from the sewn workpiece; a primary nozzle disposed laterally in the extension of the side edge of the needle plate and arranged to produce a first air blow substantially perpendicular to the sewing direction for causing reversal of the end chain thereby orienting it in a direction opposite that of sewing; a grip member operatively located before a front edge of the needle plate for retaining the reversed end chain upon the action of the primary nozzle.
The invention also relates to a process for reversing an end chain put into practice by the above device, said process being of the type comprising the steps of: drawing in and cutting the end chain in a suction chain-cutting member operating in the extension of a side edge of the needle plate, downstream of said needle plate with reference to the sewing direction; reversing the cut chain over the needle plate through 180.degree. by a first blow substantially perpendicular to the sewing direction; holding the cut chain end at a front edge of the needle plate.
2. Prior Art
It is known that in sewing machines of the trimming type for industrial use adapted to carry out a two-needle overedging, the seam is formed by interlacing of several threads respectively carried by the needles, a lower looper and an upper looper.
Interlacing of said threads occurs about a pair of tabs formed in the needle plate, extending parallel to the sewing direction and exhibiting respective free ends oriented in the workpiece feed direction. More particularly, a primary tab is provided which is formed close to a side edge of the needle plate, as well as an auxiliary tab disposed parallel to and in side by side relation with the primary tab. Defined between the primary and auxiliary tabs is a first notch in which when the machine is in operation, one of the needles is alternately inserted and exactly the needle identified as "inner needle", that is the needle located more close to the side edge of the needle plate. The other needle or "outer needle" altenately enters a second notch disposed alongside the auxiliary tab on the opposite side from the first notch.
During the stitching step, the workpiece edges are passed through by the needles and interlooped in the stitches as they are formed about the primary and auxiliary tabs.
When, at the end of the sewing operation, the end edge of the workpiece surpasses the sewing area in the provisional step preceding the machine stopping, an end-of-stitching chain is formed that consists of chain stitches not interlooped in the workpiece.
A suction chain-cutting member operatively disposed in the extension of the side edge of the needle plate, downstream of said plate, carries out suction and cutting of the chain, so as to separate it from the sewn workpiece. The cut chain length extending from the primary and auxiliary tabs, must be then reversed over the needle plate in a direction opposite the stitching direction, so that it may be arranged for insertion in the initial length of the following stitching made on a new workpiece edge. To this end, combined with the chain-cutting member is a nozzle designed to eject an air blow oriented perpendicular to the sewing direction so as to cause the chain reversal by rotation through 180.degree. in a substantially horizontal plane. The reversed chain end is engaged by a grip member holding it against the front edge of the needle plate, until starting of a new stitching.
The foregoing being stated, it is noted that in sewing machines set up for carrying out a two-needle overedge stitching the presence of two tabs in the needle plate creates some problems in that, on reversal, the chain could undesirably get entrapped in the auxiliary tab and, at the end of reversal, take an inadequate position in view of its correct insertion in the initial length of the following stitching.
In order to solve this problem, many devices have been developed that carry out retraction of the auxiliary tab in the opposite way with respect to the sewing direction on stopping of stitching, before the chain reversal is operated.
For example, one of these devices is described in European Patent No. 0037444.
In this device the auxiliary tab is located at the end of a drive rod slidably guided in a longitudinal groove formed in the needle plate. An actuator driven by an electronic control unit interlocked to the sewing machine, acts on the drive rod for retracting the auxiliary tab in a direction opposite that of sewing, during the stopping step of the sewing machine. In this manner, the auxiliary tab is slipped off the stitches formed thereon, which ensures that at the end of reversal said chain can extend on the needle plate starting from the extremity of the first notch disposed alongside the primary tab without the risk that it may undesirably step over the auxiliary tab or get entrapped thereby.
As can be easily supposed, the necessity of arranging different kinematic mechanisms for driving the forward and backward movement of the auxiliary tab in synchronism with the operation of the sewing machine involves important problems in terms of structural complexity. It is necessary in fact to arrange several moving mechanical members within the limited room available under the needle plate.
In addition, the presence of these mechanical members makes it practically unfeasible or at all events very hard to carry out the replacement of the needle plate for making the sewing machine adapted to execute stitchings different from the two-needle overedging one.
In fact the removal and replacement of the needle plate requires the complete disassembling of the device controlling the auxiliary tab movements or at least disabling of said device by removal of several mechanical parts.
it is also to be considered that the space taken up by the members designed to control the movements of the auxiliary tab cannot be utilized for the installation of possible other auxiliary devices designed to adapt the machine to execute particular workings.